In vehicles, a transmission is arranged between an internal combustion engine and driving wheels. The transmission changes a driving force of the driving wheels and a running speed in accordance with running conditions of the vehicle which change in a wide range, thereby making the performance of the internal combustion engine sufficiently effective. There is known a continuous variable transmission in which a pulley has a fixed pulley member fixed to a rotary shaft and a movable pulley member attached to a rotary shaft so as to move toward and away from the fixed pulley member, and by increasing and decreasing the width of a groove which is formed between both of the pulley members, a rotational radius of a belt which is reeved to the pulley is respectively decreased and increased, thereby changing the belt ratio. Such a continuous variable transmission has been disclosed in, for instance, JP-A-57-186656, JP-A-59-43249, JP-A-59-77159, and JP-A-61-233256.
In the control of such continuous variable transmission, when a final target engine rotational speed changes during ordinary vehicle running, if the difference between target engine rotational speed and the final target engine rotational speed is equal to or larger than a predetermined value, a rate limit control operation is executed, in which the final target engine rotational speed is changed on the basis of a predetermined rate limit value. Such rate limit control is disclosed in U.S. Pat. No. 5,012,697. However, when the vehicle running mode is changed during running or when a throttle opening degree THR is increased to a value near the fully open position, the required change in final target engine rotational speed is large, so that there occurs an inconvenience in that it is impossible to cope with such a situation using the ordinary rate limit value.
Therefore, the applicant of the present invention has already developed an apparatus for controlling the rotational speed of a continuous variable transmission in which when the running mode was changed during running or when the throttle opening degree THR was increased to a value near the fully open position, there is executed a transient control to set a time change ratio (as a rate limit value) to be larger than a predetermined time change ratio (as a predetermined rate limit value), and the final target engine rotational speed is changed, thereby improving the response speed of the engine rotational speed (Japanese Patent Application No. 63-302733).
However, in the conventional method of controlling a continuous variable transmission, there is an inconvenience in that when the actual throttle opening degree THR is equal to or larger than a predetermined throttle opening degree trigger value which is determined on the basis of vehicle velocity and throttle opening degree, the throttle transient control is started, so that the will of the driver is not reflected in the control of the vehicle speed.
For instance, in running on high ground (high elevations), since the engine torque decreases due to the influence of low atmospheric pressure, if the engine torque is not held to a high value by increasing the throttle opening degree, running similar to that at lower elevations cannot be performed. Thus, even in a situation in which the throttle transient control is unnecessary, there is a problem in that since the throttle opening degree crosses the throttle opening degree trigger value, the throttle transient control is executed in spite of the will of the driver. On the other hand, there is a problem in that in the running on high ground, in order to achieve adequate engine torque, the throttle opening degree will nearly always exceed the throttle opening degree trigger value. Thus, the actual throttle opening degree cannot cross the throttle opening degree trigger value, so that the throttle transient control is not executed even in a situation in which it is necessary.
Such problems occur not only in the running on a high ground but also when running on slopes, during high speed running, or the like. There is also a problem such that if the throttle opening degree trigger value is set to a predetermined value, when the throttle opening degree THR has slightly exceeded the throttle opening degree trigger value, the engine rotational speed instantaneously rises.
Therefore, there occur inconveniences such as improper execution of the throttle transient control, excessive engine rotational speed, and the like, so that the will of the driver is not reflected in the rotational speed control and a feeling of physical disorder is given to the driver.
If a predetermined trigger value is set and used irrespective of the running state of the vehicle, since the throttle opening degree THR is small in a low velocity range but large in a high velocity range, it is difficult to actuate throttle transient control using a single trigger value, making it difficult to achieve the throttle transient effect at all possible velocities.
Further, unlike the case of the low velocity range, since the throttle opening degree THR is large in a high velocity range, the predetermined throttle transient control is executed frequently, and more than it is needed, such that the running state deteriorates.